Effect of hypoxia on cerebral metabolites measured by proton nuclear magnetic resonance spectroscopy in rats

Untargeted metabolomics analysis on kidney tissues from mice reveals potential hypoxia biomarkers

Chronic hypoxia may have a huge impact on the cardiovascular and renal systems. Advancements in microscopy, metabolomics, and bioinformatics provide opportunities to identify new biomarkers. In this study, we aimed at elucidating the metabolic alterations in kidney tissues induced by chronic hypoxia using untargeted metabolomic analyses. Reverse phase ultrahigh performance liquid chromatography-mass spectroscopy/mass spectroscopy (RP-UPLC-MS/MS) and hydrophilic interaction liquid chromatography (HILIC)-UPLC-MS/MS methods with positive and negative ion mode electrospray ionization were used for metabolic profiling. The metabolomic profiling revealed an increase in metabolites related to carnitine synthesis and purine metabolism. Additionally, there was a notable increase in bilirubin. Heme, N-acetyl-L-aspartic acid, thyroxine, and 3-beta-Hydroxy-5-cholestenoate were found to be significantly downregulated. 3-beta-Hydroxy-5-cholestenoate was downregulated more significantly in male than female kidneys. Trichome Staining also showed remarkable kidney fibrosis in mice subjected to chronic hypoxia. Our study offers potential intracellular metabolite signatures for hypoxic kidneys.

Neuronal Damage after Ischemic Injury in the Middle Cerebral Arterial Territory: Deep Watershed versus Territorial Infarction at MR Perfusion and Spectroscopic Imaging

PURPOSE

To determine the temporal patterns of neuronal injury between infarction subtypes and their possible association with changes in cerebral blood volume (CBV).

MATERIALS AND METHODS

Twenty-five patients with ischemic injuries of middle cerebral arterial territories and receiving only conservative treatments were classified into territorial infarction (TI) (n = 16) and watershed infarction (WI) (n = 9) groups and were prospectively evaluated with longitudinal magnetic resonance (MR) examinations. Each patient underwent as many as five MR studies at various stroke stages following stroke symptom onset. Dynamic susceptibility-weighted contrast material-enhanced MR imaging was performed to yield the relative CBV (rCBV). Chemical shift imaging was used to measure the relative levels of N-acetylaspartate (NAA) and lactate of the ischemic brain tissue. Repeated-measures analysis of variance was used to examine the statistical significance in evolutional differences between TI and WI.

RESULTS

For patients with TI, rCBV followed a progressively increasing pattern, from initial low values (0.46 +/- 0.28 [SD]) to peak high values (1.23 +/- 0.34) at early chronic stage. Relative NAA level decreased to 0.40 +/- 0.24 during acute stroke and was lost completely 4 days after ictus. Patients with WI showed consistently high rCBV throughout all stages, with residual relative NAA level (0.53 +/- 0.25) even at 1 month after symptom onset. Relative lactate level of patients with TI was significantly higher than that of patients with WI at the acute stage (P <.01). Differences in the temporal changes of both rCBV and brain metabolites between TI and WI were significant (P <.01).

CONCLUSION

The different temporal patterns for stroke progression in TI and WI are associated with different evolutions of hemodynamics and neuronal injury.

Lactate accumulation during moderate hypoxic hypoxia in neocortical rat brain

Neocortical metabolism was studied during moderate hypoxic hypoxia, reoxygenation, and postmortem periods in anesthetized normocapnic rats using 1H nuclear magnetic resonance (NMR) spectroscopic imaging. Rats were prepared with unilateral common carotid occlusion to determine the ipsilateral metabolic effects of inadequate cerebral blood flow (CBF) response to hypoxia. No difference in brain metabolism between the two hemispheres was found during the control period. Hypoxic hypoxia (PaO2 = 54.1 +/- 5.8 mm Hg) resulted in a significant rise in neocortical lactate peak in both hemispheres, with an additional marked rise in the clamped side compared to the unclamped side (53 +/- 27 vs. 22 +/- 13% of postmortem value, p < 0.001). These lactate changes were not reversible within 30 min of reoxygenation in the clamped hemisphere. No changes in neocortical lactate peak were observed while elevating arterial lactate via intravenous lactate infusion without hypoxia. In addition, hypoxic hypoxia resulted in an apparent decrease in neocortical water and N-acetyl aspartate (NAA) signals, which were related to a shortening in T2 relaxation times. It is concluded that neocortical lactate is an early metabolic indicator during moderate hypoxic hypoxia in normocapnic conditions.

Hypoxic pretreatment protects against neuronal damage of the rat hippocampus induced by severe hypoxia

The present study investigates whether under conditions of successive hypoxic exposures pretreatment with mild (15% O(2)) or moderate (10% O(2)) hypoxia, protects hippocampal neurones against damage induced by severe (3% O(2)) hypoxia. The ultrastructural findings were also correlated with regional superoxide dismutase (SOD) activity changes. In unpretreated rats severe hypoxia induced ultrastructural changes consistent with the aspects of delayed neuronal death (DND). However, in preexposed animals hippocampal damage was attenuated in an inversely proportional way with the severity of the hypoxic pretreatment. The ultrastructural hypoxic tolerance findings were also closely related to increased regional SOD activity levels. Thus the activation of the endogenous antioxidant defense by hypoxic preconditioning, protects against hippocampal damage induced by severe hypoxia. The eventual contribution of increased endogenous adenosine and/or reduced excitotoxicity to induce hypoxic tolerance is discussed.

Mapping of lactate and N-acetyl-L-aspartate predicts infarction during acute focal ischemia: in vivo 1H magnetic resonance spectroscopy in rats

The time course, anatomic distribution, and extent of changes in cerebral lactate, N-acetyl-L-aspartate (NAA), and other metabolite levels determined by three-dimensional in vivo 1H magnetic resonance spectroscopy and single-voxel spectral analysis after middle cerebral artery occlusion in rats. Increased lactate was detected in the central ischemic region within 1.3 hours after the onset of permanent occlusion (n = 22) or 0.5 hour after the onset of 1 hour of temporary occlusion and then reperfusion (n = 8). Permanent occlusion resulted in persistent lactate elevation and a 25.4 +/- 4.1% reduction in the NAA peak after 1.3 hours; NAA was almost completely depleted after 24 hours. Results also demonstrated delayed depletion of all other magnetic resonance spectroscopy-visible 1H metabolites, including creatine, choline, and glutamate, after permanent occlusion. After 1 hour of temporary focal ischemia, lactate returned to nearly normal levels within 0.4 hour after the onset of reperfusion; at 72 hours, a recurrent increase in lactate and a new decrease in NAA were observed, suggesting delayed tissue injury. Histological analysis, performed in 10 rats, demonstrated infarcts that corresponded in distribution to regions of NAA depletion at 72 hours. These findings indicate that lactate elevation is a sensitive early marker of ischemia; however, temporary recovery of lactate accumulation after reperfusion did not predict sustained metabolic recovery. In contrast, NAA depletion within 1.3 hours after the onset of ischemia identified central ischemic regions that were destined for infarction. Potential clinical applications include selection and monitoring of therapeutic intervention, as well as prediction of outcome, in patients with acute stroke.

Proton magnetic resonance spectroscopy in children with acute central nervous system injury

Single voxel proton magnetic resonance spectroscopy (1H-MRS) was used in 30 infants and children with acute central nervous system injuries to determine the value of changes in specific metabolite ratios in predicting outcome. The mean age of all patients was 38 +/- 52 months and the mean time of study after insult was 7 +/- 5 days. 1H-MRS was determined in the occipital gray and parietal white matter (8 cm3 volume, STEAM sequence with TE = 20 ms, TR = 3,000 ms). Data were expressed as ratios of different metabolite peak areas including N-acetylaspartate (NA), choline-containing compounds (Ch), creatine and phosphocreatine (Cr), and lactate (Lac). Statistically significant differences were observed when patients with good/moderate (G/M) outcomes (n = 17; mean age: 46 months) were compared to patients with bad outcomes (n = 10; mean age: 26 months). NA/Cr and NA/Ch were significantly lower in the bad outcome group (NA/Cr = 1.15 +/- 0.38; NA/Ch = 1.18 +/- 0.52) compared to the G/M group (NA/Cr = 1.41 +/- 0.28, P < .05; NA/Ch = 1.98 +/- 0.81, P < .01). Lactate was present in 80% of bad outcome patients and in none of the G/M group (P < .0001). Using a linear discriminant analysis and combining 4 clinical variables (Glasgow Coma Scale score, initial pH and glucose, number of days unconscious at time of 1H-MRS) allows classification of 94% of patients into their correct outcome group. Use of spectroscopy variables (NA/Cr, NA/Ch, Ch/Cr, presence of lactate) alone correctly classified 81% of patients. The combination of clinical and 1H-MRS variables correctly classified 100% of patients. Our findings suggest that 1H-MRS adds information which, in combination with clinical examination, may be useful in outcome assessment in children with serious acute central nervous system injury.

Proton spectroscopy of the neonatal brain following hypoxic-ischaemic injury

Proton magnetic resonance spectroscopy was used to examine, within the first month of life, the brains of 11 infants born at term--10 with signs of hypoxic-ischaemic encephalopathy (HIE) and one who was neurologically normal at birth. All the infants had peak resonances on their spectra which could be assigned to N-acetyl-aspartase (NAA), choline-containing compounds (Cho) and creatine plus phosphocreatine (Cr). When neurodevelopmental outcome at one year was correlated with initial spectroscopy findings, the NAA/Cho and NAA/Cr ratios reflected clinical outcome. This study suggests that proton spectroscopy not only provides new information about biochemical changes occurring in the brains of infants with HIE, but also may help to predict outcome within the first month of life.

Energy reserves and utilization rates in developing brain measured in vivo by 31P and 1H nuclear magnetic resonance spectroscopy

Age-related changes in cerebral energy utilization were examined in swine, a species whose maximal rate of development is known to occur in the perinatal period. Interleaved in vivo 31P and 1H nuclear magnetic resonance spectroscopy was used to measure the rates of change in cerebral concentrations of phosphocreatine (PCr), nucleoside triphosphates, and lactate following complete ischemia, induced via cardiac arrest, in a total of 19 newborn, 10-day-old, and 1-month-old piglets. Preischemic concentrations of these three metabolites plus glucose and glycogen were determined in a separate experiment on 12 piglets whose brains were funnel-frozen in situ. The rate constants for the PCr and ATP decline and lactate increase were determined by nonlinear regression fits to the experimental data, assuming first-order kinetics. The rate constants and preischemic metabolite concentrations were used to calculate the initial flux of high-energy phosphate equivalents (approximately P), which was used as an estimate of cerebral energy utilization at the point when ischemia was initiated. Cerebral energy utilization equaled 6.5 +/- 1.9, 9.5 +/- 3.2, and 15.1 +/- 3.2 mumol approximately P/g/min in newborn, 10-day-old, and 1-month-old piglets, respectively. Within each age group the energy utilization rate was not altered by hyperglycemia-induced increases in cerebral energy reserves, but during hypoglycemia cerebral energy utilization rates decrease. The slope of approximately P versus time decreased with the duration of ischemia, indicating that cerebral energy utilization rates decrease after the first few minutes of ischemia. Newborn piglets had higher cerebral energy utilization rates compared with literature values for newborn rats and mice. This is consistent with the concept that newborns from a species with a perinatal stage of maximal growth and development will have higher cerebral energy demands compared with newborns from a species such as rodents, whose maximal growth occurs postnatally. However, this conclusion remains tentative because literature cerebral utilization rates estimated from the initial slope of approximately P-versus-time plots tend to underestimate the true rate, since the assumption of continued linearity may not be valid for the interval chosen.